Fastener with thread lock

ABSTRACT

A threaded fastener that self locks as it is tightened to a fastened object. The thread includes one or more portions that are shifted, resulting in one or more discontinuities in the thread. Each shifted portion of thread is fully formed at the discontinuity. The shifted portions will deform a relatively softer female thread, causing the deformed thread to move up and over the end faces of the shifted portions and to pack up in front of the end faces to prevent any significant loosening of the connection.

This invention relates to threaded fasteners and, in particular, toself-locking threaded fasteners having discontinuous threads.

BACKGROUND OF THE INVENTION

Threaded fasteners are useful in a wide variety of applications whereload bearing conditions are required and where high vibrations canresult in the loosening or release of fasteners that are notself-locking. Most conventional self-locking fasteners requiredeformation of the mating nut with a special tool to obtain thenecessary mechanical lock. This process, however, can remove theprotective finish from the nut, allowing corrosion to take place.Furthermore, a special nut or collar must be used.

Other fasteners achieve self-locking by slitting or displacing the crestof the thread. For example, in U.S. Pat. No. 2,177,005 to Purtell, a setscrew is described wherein the crest of the thread is slit, forming aresilient lip that yields upon tightening of the screw, but whichresists loosening of the screw by pressing tightly against the upperside face of the female thread. The lip may also be provided with asharp edge that digs deeply into the side face of the female thread uponreversal of the screw. This type of fastener, however, does not resultin adequate locking in certain instances. In one embodiment, thefastener relies on friction alone to prevent loosening. In thealternative embodiment, a mechanical lock is created upon loosening ofthe screw. This, however, results in the release of at least part of thepreload that has been placed on the fastened connection.

A similar fastener is shown in U.S. Pat. No. 3,124,188 to Muenchingerwhich describes a screw having a helical thread with disalignedsegments. The patent states that the screw may be tightened with aminimum of additional resistance caused by the disaligned segments,whereas, when torque is applied in the opposite sense, to remove thescrew, the upper or leading end of each segment will tend to bite intothe abutting flange of the female thread with which it is associated,thus resisting removal of the screw. The patent recommends casehardening or other treatment of the screw to impart resilience to thedisaligned segments. Accordingly, this fastener also suffers from thedisadvantages noted above, namely, it locks upon loosening of the screw,permitting a release of the preload intended to be applied to thefastened joint.

It should, therefore, be appreciated that there still is a need for athreaded fastener that achieves a mechanical lock, without loss ofpreload, and without the use of special tools or special procedures forachieving such a lock. The present invention satisfies this need.

SUMMARY OF THE INVENTION

The present invention is embodied in a threaded fastener that self-locksas it is being tightened to a fastened object. The fastener destroys thefemale thread during final setting and therefore is used for permanentinstallations like a solid rivet. Such a fastener includes a threadedshank that has a bearing end and a lead end, the lead end being the endof the shank that is threaded first. The thread includes one or moreportions that are shifted, resulting in one or more discontinuities,respectively, in the thread.

A feature of the present invention is that each shifted portion ofthread is fully formed at the discontinuity and is shifted toward thebearing end of the shank relative to the next succeeding portion of thethread. By fully formed, it is meant that the root of the shiftedportion of thread is essentially as fully developed as the root of thenon-shifted portion of thread. As will be seen, the fully formed threadof each shifted portion has an exposed end face at the discontinuity,that acts as a non-resilient buttress that locks the fastener as theconnection is tightened.

The threaded shank of the present invention may be used with aconventional nut, a swaging collar, or may be threaded into a tappedhole. Preferably, the nut or collar, or the material defining the tappedhole, are made of a relatively softer material than the threaded shank.For example, in the case of a threaded shank used with a softer nut forfastening to a workpiece, the nut is threaded onto the shank and as thenut is tightened to the workpiece, a compression force is applied by thenut to the thread of the shank, including the shifted portions ofthread. If a sufficient compression load is applied to the shiftedportions, the outer edge of the end face of each shifted portion, ateach respective discontinuity, will cut into the nut. Upon final settingor tightening of the fastener, the shifted portions will deform the nutcausing the deformed nut material to move up and over the end faces ofthe shifted portions and pack up in front of the buttresses to preventany significant loosening of the connection. A tight,vibration-resistant, mechanical lock is thus obtained.

Another feature of the present invention is the particular location ofthe shifted portions on the thread such that the discontinuities occurwithin the first few revolutions of thread at the bearing end of thethreaded shank. It is known that the compression load applied to athreaded fastener is taken up by the first few revolutions of thethreaded shank. In particular, the first revolution may take up as muchas approximately 35% of the load once the fastener is properly set.Accordingly, placing the buttresses of the shifted portions in the firstfew revolutions of the thread, and especially placing a buttress in thefirst revolution, ensures that a sufficient force will develop at theouter edges of the shifted portions to cause deformation of the nut.

The present invention is also embodied in a die set that is used inmanufacturing the above-described threaded fasteners. Both dies of thedie set have lands and grooves that are set at a predetermined pitch.One of the dies, however, is provided with an insert near its "exit" endhaving a separate set of lands and grooves. The insert is located in thedie so as to receive an already fully formed threaded shank. The landsand grooves of the insert then shift the first few revolutions of threadof the fully formed shank to create the above described shifted portionsof thread. The insert is close enough to the exit end so that theshifted portions are not re-rolled prior to exiting the die set. Thegrooves of the insert are deep enough to ensure that the shiftedportions on the threaded shank are fully formed at the discontinuities.

Other features and advantages of the present invention will becomeapparent from the following description of the preferred embodiment,taken in conjunction with the accompanying drawings, which illustrate,by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view of a threaded shank having a discontinuousthread in accordance with the present invention.

FIG. 2A is an enlarged perspective view of the circled area A in FIG. 1.

FIG. 2B is an enlarged partial view of FIG. 1 as viewed from line B--B.

FIG. 3 is a sectional view of a nut threaded onto a threaded shank inaccordance with the present invention.

FIG. 4 is a sectional view of a threaded shank in accordance with thepresent invention threaded into a tapped hole.

FIG. 5 is a sectional view of a threaded shank in accordance with thepresent invention used with a swaged collar, the collar shown in apreswaged, undeformed condition and in a swaged, deformed condition.

FIG. 6 is a perspective view of a die set in accordance with the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A threaded fastener embodying the features of the present invention isshown in FIGS. 1-5. FIG. 1 illustrates a pin 12 having the improvedthread lock.

The pin 12 includes a head 22 and a shank 24, the shank having a bearingend 26 near the head of the pin and a lead end 28 that is the end of theshank that is threaded first. Between the bearing end and the lead endof the shank is a helical thread 30 made up of a number of revolutions32. The thread includes a crest portion 34 and a root portion 36, theroot portions of axially adjacent revolutions of the thread forming athread groove 38. Each revolution of the thread also has a pressure sideflank 40 and a lead side flank 42.

For purposes of the present description, the thread may be said to bedivided into two portions (see FIG. 4), a continuous normal portion 44of thread beginning at and extending from the lead end 28 of the shank,and a discontinuous portion of thread 46 having a combination of shiftedportions 48 of thread and partial normal portions 50 of thread.Preferably, for each revolution 32 of thread in the discontinuousportion, the shifted portion 48 represents approximately one-quarter ofthe revolution and the partial normal portion 50 represents theremainder. Each shifted portion is continuous with the next precedingrevolution of thread (i.e., in the direction of the lead end of theshank), but is discontinuous, at a buttress end 54 thereof, with thenext succeeding revolution of thread (i.e., in the direction of thebearing end of the shank). At each discontinuity, a front face 56 at thebuttress end of the shifted portion opposes a rear face 58 of the nextsucceeding revolution creating an interface 60 (see FIGS. 2A and 2B).

The pitch of the thread 30, except for the shifted portions 48, is setat a predetermined angle. The pitch of the shifted portions is set at agreater angle such that the buttress end 54 of each shifted portion islocated above the next succeeding revolution at the interface 60.Preferably, the buttress end of each shifted portion is shifted relativeto the partial normal portion 50 of the thread approximately one-quarterpitch at the discontinuity.

With reference now to FIGS. 2A and 2B, the thread lock of the presentinvention is shown in greater detail. In particular, it is seen that theshifted portion 48 is a fully formed thread. By fully formed, it ismeant that the root of the shifted portion of thread is essentially asfully developed as the root of the non-shifted portion of thread. Thefront face of the shifted portion has an outer edge 62 that, as will beseen, acts as a cutting edge that gouges into the female thread of anut, collar or a tapped hole.

With reference to FIG. 3, the above-described pin 12 and a nut 16 havingfemale threads and grooves are shown fastened to a workpiece 14. The pinis preferably made of hardenable material and the nut is made of arelatively softer material, such as aluminum, stainless steel ortitanium. As the nut is tightened, it moves up on the pin and out of thepage as viewed in FIG. 3. During initial fastening, the grooves of thenut are sufficiently wide to simultaneously accommodate the shiftedportions 48 and the partial normal portions 50 of the thread at thediscontinuities. Once the nut bears on the workpiece, however, acompression force will be applied by the nut onto the pressure sideflanks 40 of the thread. Upon final setting of the fastener, significantforce will build up on the pressure side flanks and, in particular, theforce will be especially concentrated along the outer edges 62 of theshifted portions, causing the outer edges to plastically deform thefemale thread. This, in turn, will cause the nut to ramp up and over theouter edges. The deformed nut material will then pack up against thefront face 56 of the shifted portion. The front face thus acts as abuttress, preventing any significant loosening of the connection.Similarly, with reference to FIG. 4, a bolt 52 is tightened in aclockwise direction (as shown by the arrow) into a tapped hole 20,causing the material forming the tapped hole to deform and pack up infront of the buttress end 54 of the shifted portion of thread.

It will be appreciated that after final setting of the fastener, thefemale thread will have been destroyed, forming a mechanical lockbetween the pin and the female thread. Of importance, is that thismechanical lock is formed during final tightening of the fastener andthus will hold the preload better than prior fasteners that have usedother forms of thread locks.

In the preferred embodiment of the present invention, the discontinuousportion 46 of thread comprises the first few revolutions of thread atthe bearing end 26 of the shank and, in particular, a discontinuity 64is located in the very first revolution of thread at the bearing end ofthe shank. This is to take advantage of the compression load acting onthe pressure side flanks of the first few revolutions of the thread atthe bearing end of the shank. The first revolution itself may take up asmuch as approximately 35% of the compression load. This load istransmitted throughout the thread length, including the outer edge 62 ofthe shifted portion. This high force will cause the outer edge toplastically deform the softer female thread. For example, applying a1600 lb. pre-load to a pin having a 0.250-28 UNJF-3A thread form, willresult in an approximate load of 560 (1600×0.35) lbs. throughout thelength of the first thread. Measuring the area of the outer edge by itslength (approximately 0.03 in.) and edge width (approximately 0.0005in.) results in a cross-sectional area of approximately 0.000015 sq. in.taking up the 560 lb. load or approximately 37 million psi. A nut madeof 2024-T6 aluminum, for example, has a bearing strength ofapproximately 129,000 psi, thus the outer edge will easily cut into thenut thread upon final setting of the fastener.

With reference now to FIG. 5, a swaged fastener 120 is shown including apin 122 having the thread lock 124 of the present invention and anunthreaded swaging collar 126. The swaging collar is shown above the pinin its preswaged, undeformed condition and also shown swaged to the pin.In the swaged condition, the collar thread is deformed to preventloosening of the connection. Swaging is accomplished by methods wellknown to persons having ordinary skill in the art.

With reference now to FIG. 6, a thread rolling die set 70 formanufacture of fasteners having the thread lock of the present inventionis shown. The die set includes an upper reciprocating die 72 having anentry end 74 and an exit end 76 and a lower stationary die 78 having anentry end 80 and an exit end 82. An unthreaded pin 84 is positioned withits shank 86 between the entry ends of the dies. In the rollingoperation, the reciprocating die is moved to the left causing the pin toroll, also to the left, between the two dies and, due to the compressionexerted upon the dies, causing the metal of the pin shank to be squeezedinto the grooves of the respective dies to form a thread upon the shank.The nature of the thread so formed is, of course, dependent upon thenature of the threading grooves with which the respective dies areprovided. Threading dies of the type here in question are customarilyprovided with their threading grooves by means of grinding operations,the slope or inclination of the grooves, and hence the threads to beformed thereby, being governed by the "grinding angle" employed duringthe grinding operation.

In the present case, the reciprocating die is shown having a rollsurface 88 that includes a set of grooves 90 that are all aligned at apredetermined angle. The stationary die also has a roll surface 92 thatincludes a set of grooves 94 corresponding to the grooves on thereciprocating die roll surface and formed at the same predeterminedangle. The width of the roll surface determines the length of the threadto be formed on the pin, whereas the length of the roll surface must besufficient to form a fully formed thread on the shank.

In order to form the shifted portion of the thread on the pin, thestationary die is provided with a notch 100 for receiving a specialinsert 102. The notch is along an outside edge 104 of the stationary dienear its exit end 82. In particular, the notch is on the side of the diethat will receive the bearing end of the pin. This is to ensure that thefirst few revolutions of the pin at the bearing end will be shifted. Thenotch is also sufficiently downstream of the entry end 80 of the die toensure that the thread of the pin is fully formed before reaching theinsert 102. Regarding the size of the notch, its width (i.e., thedistance inwardly from side edge 104) will determine the number ofrevolutions of thread on the pin that will be affected and (its length(i.e., the distance in the direction of rolling) will determine thecircumferential length of the shifted portions.

The insert 102 for shifting the fully formed thread of the pin isreceived within the notch of the stationary die. The insert has a rollsurface 106 that includes an entry side 108 (i.e., the side of theinsert that the pin will roll onto first during the rolling process), anexit side 110 (i.e., the side of the insert that the pin will exit fromduring the rolling process) and a set of grooves 112. The set of grooveson the roll surface of the insert are ground at a greater angle than theset of grooves on the roll surface 92 of the stationary die 78. Theinsert set of grooves are disposed such that the stationary set ofgrooves and the insert set of grooves are disaligned at the entry sideof the insert, but aligned at the exit side of the insert. The depth ofthe grooves on the roll surface of the insert is sufficient to createfully formed shifted portions of thread on the pin.

In the preferred embodiment, the stationary die has a support portion114 on the exit side 110 of the insert and a window portion 116 betweenthe support portion and the exit end 82 of the stationary die. Thesupport portion acts as a downstream support for the insert. Preferably,the support portion is integral with the stationary die and has agrooved roll surface that is continuous with and has the same pitch asthe grooves of the roll surface of the stationary die. This latterfeature improves the transition between shifted portions and normalportions of thread on the pin.

The window portion 116 is an angled cutout area on the stationary diethat prevents the shifted portions of the thread from becoming re-rolledas the pin exits the die set. In this regard, it is important that thecombined length of the insert 102 and the support portion 114 be lessthan 180° of the circumference of the pin to prevent re-rolling of theshifted portions. Alternatively, if the reciprocating die 72 is providedwith a window or cutout area at its point of correspondence with theinsert and support portion during rolling, then the insert and supportportion may have a combined length of up to 360° of the circumference ofthe pin.

During the thread rolling process, an unthreaded pin will be inserted atthe entry end 74, 80 of the die set. Prior to reaching the insert, thethread of the pin will be fully formed. The insert will then cause anabrupt shift in the first few revolutions of the thread at the bearingend of the shank, creating a discontinuity in each of those threadrevolutions. The length of the insert (i.e., from entry side to exitside) determines the circumferential length of the shifted portions.Finally, before the pin rotates a full revolution after striking theentry side of the insert, the pin will enter the window portion of thedie and subsequently exit the die set.

It should be appreciated from the foregoing description that the presentinvention provides a fastener having an improved thread lock thatachieves a mechanical, vibration-resistant, lock during final tighteningof the fastener and that more effectively prevents a release of thepreload applied to the fastened joint. The fastener includes a threadedshank wherein each of the first few revolutions of thread have fullyformed shifted portions forming discontinuities in the thread. The shankmay be used with a nut or a collar made of a relatively softer material,or may be inserted into a tapped hole formed of a softer material or,may even be used with a swaged collar that is swaged onto the threadedshank.

It will, of course, be understood that modifications to the presentlypreferred embodiment will be apparent to those skilled in the art.Consequently, the scope of the present invention should not be limitedby the particular embodiment discussed above, but should be defined onlyby the claims set forth below and equivalents thereof.

I claim:
 1. A threaded fastener, comprising:a shank having a bearing endand a lad end, the lead end being the end of the shank that is threadedfirst; a helical thread extending between said ends, the thread having acrest and a root, the roots of axially adjacent revolutions of thethread forming a thread groove; and said helical thread having a portionthat, before installing the fastener, is shifted toward the bearing endof the shank resulting in a discontinuity of the thread, the shiftedportion of thread having a buttress end that is fully formed from crestto root at the discontinuity.
 2. The threaded fastener of claim 1,wherein the shifted portion is continuous with a next preceding portionof thread that is not shifted.
 3. The threaded fastener of claim 1,wherein the discontinuity is located in the first revolution of threadat the bearing end of the shank.
 4. The threaded fastener of claim 1,wherein the length of the shifted portion of thread is approximatelyone-quarter the circumference of the shank.
 5. The threaded fastener ofclaim 1, wherein the buttress end of the shifted portion is offsetapproximately one-quarter pitch relative to a next succeeding portion ofthread that is not shifted.
 6. A threaded fastener, comprising:a shankhaving a bearing end and a lead end, the lead end being the end of theshank that is threaded first; a helical thread extending between saidends, the thread having a crest and a root, the roots of axiallyadjacent revolutions of the thread forming a thread groove; and saidhelical thread having a plurality of portions that, before installingthe fastener, are shifted toward the bearing end of the shank andresulting in respective discontinuities of the thread, each shiftedportion of thread having a buttress end that is fully formed from crestto root at its respective discontinuity.
 7. The threaded fastener ofclaim 6, wherein each shifted portion is continuous with a nextpreceding portion of thread that is not shifted.
 8. The threadedfastener of claim 7, wherein each of a plurality of revolutions ofthread has one of said discontinuities.
 9. The threaded fastener ofclaim 8; wherein the plurality of revolutions includes the firstrevolution of thread at the bearing end of the shank.
 10. The threadedfastener of claim 9, wherein the discontinuities are axially aligned andeach located in consecutive revolutions of thread, respectively,
 11. Athreaded fastener, comprising:a shank having a bearing end and a leadend, the lead end being the end of the shank that is threaded first; ahelical thread extending between said ends, the thread having a crestand a root, the roots of axially adjacent revolutions of the threadforming a thread groove; said helical thread having a first plurality ofrevolutions beginning with the first revolution at the bearing end ofthe shank, each of said first plurality of revolutions having adiscontinuity that is formed by a portion of thread that, beforeinstalling the fastener, is shifted toward the bearing end of the shank,each shifted portion of thread having a buttress end that is fullyformed from crest to root at its respective discontinuity; and a femalemember for engaging the thread of the shank, said female member beingmade of a relatively softer material than the thread of the shank. 12.The threaded fastener of claim 11 wherein said female member is anunthreaded swaging collar.
 13. The threaded fastener of claim 11,wherein said female member is a nut having a female thread to engage thethread of the shank.
 14. A threaded fastener, comprising:a shank havinga bearing end and a lead end, the lead end being the end of the shankthat is threaded first; a helical thread extending between said ends,the thread having a crest and a root, the roots of axially adjacentrevolutions of the thread forming a thread groove; and said helicalthread having a portion that, before installing the fastener, is shiftedtoward the bearing end of the shank relative to a next succeedingportion of thread in the direction of the bearing end of the shank,resulting in a discontinuity of the thread between the shifted portionand the next succeeding portion, the shifted portion of thread having abuttress end that is fully formed from crest to root at thediscontinuity.
 15. The threaded fastener of claim 14, wherein theshifted portion is continuous with a next preceding portion of threadthat is not shifted.
 16. The threaded fastener of claim 15, wherein thelength of the shifted portion of thread is approximately one-quarter thecircumference of the shank.
 17. The threaded fastener of claim 15,wherein the buttress end of the shifted portion is offset approximatelyone-quarter pitch relative to a next succeeding portion of thread thatis not shifted.
 18. The threaded fastener of claim 14 wherein saidhelical thread has a plurality of portions that, before installing thefastener, are shifted toward the bearing end of the shank, resulting inrespective discontinuities of the thread, each shifted portion of threadhaving a buttress end that is fully formed from crest to root at itsrespective discontinuity.
 19. The threaded fastener of claim 18, whereineach shifted portion is continuous with a next preceding portion ofthread that is not shifted.
 20. The threaded fastener of claim 19,wherein the discontinuities are axially aligned and each located inconsecutive revolutions of thread, respectively.